Field of the Invention
The invention relates to a turbine shaft, in particular for a steam turbine, which is oriented along an axis of rotation and has a first axially oriented region with a maximum radius R1 and a second axially oriented region adjoining the first axially oriented region and having a maximum radius R2. U.S. Pat. No. 3,767,390 describes a martensitic special steel for high-temperature applications, for example for producing steam-turbine blades or bolts for connecting two halves of a steam-turbine casing. That steel preferably has a content (all of the following data are given in per cent by weight) of 12% chromium and approximately 0.3% niobium. The addition of niobium is intended to increase the creep rupture strength and largely remove xcex4-ferrite from the steel. In a preferred embodiment, the steel described therein has, as further alloying constituents, 0.25% Co, 4% Mn, 0.35% Si, 0.75% Ni, 1.0% Mo, 1.0% W, 0.3% V, 0.75%o N, as well as a remainder of iron and impurities of sulfur, phosphorus and nitrogen.
An article entitled xe2x80x9cDevelopment and Production of High Purity 9Cr1MoV Steel for High Pressurexe2x80x94Low Pressure Rotor Shaftxe2x80x9d by T. Azuma, Y. Tanaka, T. Ishiguro, H. Yoshita and Y. Iketa, in Conference Proceedings of Third International Turbine Conference, 25-27 April 1995, Civic Centre, Newcastle upon Tyne, Great Britain, xe2x80x9cMaterials Engineering in Turbines and Compressorsxe2x80x9d, publisher A. Strang, pages 201 to 210, describes a steel for a combined high-pressure and low-pressure steam-turbine shaft. The steel is said to be suitable for the production of such a turbine shaft from a single material. In a preferred embodiment, it has a composition of 9.8% chromium, 1.3% nickel, 0.16% carbon, less than 0.1% silicon, less than 0.1% manganese, 1.4% molybdenum, 0.21% vanadium, 0.05% niobium, 0.04% nitrogen, and a remainder of iron and impurities of phosphorus, sulfur, aluminum, arsenic, tin, antimony.
The high-pressure part of the turbine shaft has a diameter of 1200 mm and the low-pressure part has a diameter of 1750 mm. The turbine shaft as a whole is produced from a blank having a diameter of 1800 mm.
It is accordingly an object of the invention to provide a turbine shaft, in particular for a steam turbine, and a method for producing a turbine shaft, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and in which the turbine shaft is suitable for high thermal stresses with a temperature profile that decreases in axial direction, with a maximum temperature of over 550xc2x0 C.
With the objects of the invention in view, there is also provided a turbine shaft, in particular for a steam turbine, oriented along an axis of rotation, comprising a first axially oriented region with a first maximum radius; a second axially oriented region adjoining the first axially oriented region and having a second maximum radius greater than the first maximum radius; the first region including a first base material for use at a first temperature, the second region including a second base material for use at a second temperature lower than the first temperature; and the base materials having an alloy steel containing 8.0% by weight to 12.0% or 12.5% by weight Cr with substantially identical austenitizing temperatures. The first base material is suitable for use at a higher temperature, in particular of over 550xc2x0 C. and the second base material is suitable for use at a lower temperature, in particular between 350xc2x0 C. and 550xc2x0 C.
In accordance with another feature of the invention, the first base material has a lower content, in percent by weight, of nickel than the second base material, in particular a nickel content which is lower by more than 0.1%. The content of nickel, in percent by weight, is between 0.1% and 1.8% for each base material, preferably 1.0% to 1.5% nickel, preferably 1.3% for the second base material, and 0.2% to 0.6% nickel for the first base material. The chromium content of the first base material, in particular for a high-pressure part of a steam turbine, is (data in percent by weight) 10% to 12% and the chromium content of the second base material, in particular for a low-pressure part of a steam turbine, is (data in percent by weight) 9.5% to 10.5%, preferably 9.8%.
In the case of a turbine shaft which has alloy steels that are different in regions but have identical austenitizing temperatures and, in the first region having a smaller cross-section, has a base material with an optionally higher chromium content and a lower nickel content than in the second region having a larger cross-section, a high hot strength, a high creep rupture strength and a sufficient fracture toughness are achieved in the first region. In the second region, high yield strength demands are fulfilled and a very good notched impact strength and fracture toughness are ensured. A required yield strength Rp02 may be, for example, around 720 MPa {square root over (m)}. The fracture toughness is, for example, about 200 MPa and, with regard to the toughness, it can be stated that the FATT is less than 25xc2x0 C. Due to the high hot strength of the first region, the latter is suitable as a high-pressure part of a combined high-pressure/low-pressure steam turbine, even at steam admission temperatures of over 550xc2x0 C. to about 650xc2x0 C. The second region is preferably suitable for use at temperature stresses of 350xc2x0 C. to about 550xc2x0 C. Different choices of the chromium and nickel content in the first region and the second region permit the high heat resistance in the first region and the toughness in the second region to be set selectively, largely independently of one another, depending on the demands placed on the materials. In contrast to a turbine shaft which is produced from a single material, there is no need to compromise between creep rupture strength in the thermally higher stressed region and toughness in the second region, which is subjected to slightly less thermal stress. In addition, as a result of having base materials of similar composition, the problem of the base materials having significantly different material properties mixing in a transition zone between the first region and the second region does not arise. Along the axis of rotation, the turbine shaft has different thermomechanical properties in regions with specifically selected different chemical compositions. In this case the regions can be produced by melting down differently alloyed electrodes by the electro-slag remelting process (ESR process).
Due to the essentially identical austenitization temperature, the material properties in the transition zone between the first region and the second region change slightly, at most. They are thus largely independent of the respective chemical composition. A similar composition of the main carbide-forming elements and main nitride-forming elements, such as C, N, V, Nb, Mo, W in the base materials results in the essentially uniform austenitizing temperature for the entire turbine shaft. This means that, in contrast to turbine shafts having significantly different base materials, the first region can be austenitized at the same temperature as the second region. A different temperature treatment, in particular for a high-pressure and low-pressure part of a steam-turbine shaft, would have a negative effect on the respective austenitizing operations.
It is now possible to produce a largely ferrite-free structure of the entire turbine shaft in one operating step.
The following stabilizing and tempering temperatures only differ from one another slightly. Moreover, using different tempering temperatures for various regions in the axial direction of the turbine shaft presents no technical problems.
In accordance with a further feature of the invention, the austenitizing temperature is in a range from 950xc2x0 C. to 1150xc2x0 C., in particular approximately 1050xc2x0 C.
In accordance with an added feature of the invention, the first base material includes (data in per cent by weight) 0 to 3% tungsten, 0 to 3% cobalt and/or 0 to 2% rhenium.
In accordance with an additional feature of the invention, the tungsten content is preferably between 2.4% and 2.7% and/or the cobalt content is preferably between 2.4% and 2.6%. The addition of rhenium makes it possible to increase the creep rupture strength.
In accordance with yet another feature of the invention, the first base material includes, as further alloying components,
(data in per cent by weight):
0% to 0.5% Mo, preferably 0.15% to 0.25%,
0.1% to 0.3% V, preferably 0.15% to 0.25%,
0.02% to 0.18% Nb, preferably 0.04% to 0.08%,
0.05% to 0.25% C, preferably 0.08% to 0.12%,
0.01% to 0.07% N, preferably 0.015% to 0.045%.
and deoxidizing elements, such as  less than 0.15% Si,  less than 0.7% Mn, preferably 0.4% to 0.6%, and a remainder of iron and possibly production-related impurities, in particular of phosphorus, antimony, tin, aluminum, arsenic and sulfur.
The first base material may be a high-purity (superclean, ultrasuperclean) alloy steel having a very low impurities content. Such alloy steels, in particular for 12% chromium steels, are described, for example, in a conference report entitled xe2x80x9cClean Steel, Super Clean Steelxe2x80x9d Mar. 6-7, 1995, Copthorne Tara Hotel, London, Great Britain, in an article entitled xe2x80x9cThe EPRI Survey on Superclean Steelsxe2x80x9d by J. Nutting, in particular in Table 1, and also in an article entitled xe2x80x9cDevelopment of Production Technology and Manufacturing Experiences with Super Clean 3,5 NiCrMoV Steelsxe2x80x9d by W. Meyer, R. Bauer and G. Zeiler in particular in the tables relating to the 12% chromium steel (Bxc3x6t550S0).
In accordance with yet a further feature of the invention, at least the first base material, i.e. the base material for the region with the smaller radius and high hot strength, includes, as a further alloying component, up to 0.03% by weight, in particular 0.005% by weight to 0.02% by weight, boron.
In accordance with yet an added feature of the invention, the second base material includes, as further alloying elements:
1.0% to 1.6% Mo, preferably 1.4%,
0.15% to 0.25% V, preferably 0.21%,
0.03% to 0.07% Nb, preferably 0.05%,
0.03% to 0.06% N, preferably 0.04%,
up to 0.1% Si,
0.1 to 0.2% C, preferably 0.16%, and up to 0.2% Mn.
In accordance with yet an additional feature of the invention, the turbine shaft is suitable for use in a steam turbine, the first region serves to receive rotor blades of a high-pressure part of the steam turbine and the second region serves to receive rotor blades of a low-pressure part of the steam turbine. In this case, during operation of the steam turbine, the high-pressure part may be subject to a steam temperature of 550xc2x0 C. to 650xc2x0 C., which requires a good hot strength of the first region, primarily in the region close to the surface.
Lower temperatures prevail in the vicinity of the axis of rotation than at the surface so that, if desired, a core region made of a base material having a lower hot strength, for example the second base material, may also be formed close to the axis in the high-pressure part. The second region, which forms the low-pressure part of the steam turbine and has a larger radius than the first region, is subject to higher mechanical stresses than the high-pressure part, in particular due to the larger low-pressure rotor blades and its own larger radius. A high toughness, in particular fracture toughness, is therefore required for the low-pressure part, which is achieved by a suitable choice of the alloying components (higher nickel content, optionally lower chromium content) of the second base material. The thermal stressing of the low-pressure part in this case is preferably below 500xc2x0 C., in particular below 480xc2x0 C. The yield strength may be over 720 MPa.
In accordance with again another feature of the invention, in view of the decreasing temperature in the turbine shaft radially in the direction of the axis of rotation in the event of a surface temperature stressing, the first region has a core region close to the axis which is surrounded by a shell region. The shell region preferably is formed of the first base material and thus has the required hot strength. The core region preferably is formed of the second base material or a third base material which also has a good hot strength. In this case the core region may be produced by electro-slag remelting of an appropriately alloyed electrode or electrodes.
The maximum radius R1 of the first region, the high-pressure part, is preferably between 350 mm and about 750 mm. The maximum radius R2 of the second region, i.e of the low-pressure part, is preferably between 700 mm and 1000 mm.
With the objects of the invention in view, there is also provided a method for producing the turbine shaft, which comprises producing the first region by melting down at least one electrode made of the first base material; and producing the second region by melting down at least one electrode made of the second base material; and simultaneously joining the first and second regions to one another. The melting may be performed, for example, by an ESR process.
The entire shaft can be produced in a single operation, in which case firstly electrodes made of the first base material and then electrodes made of the second base material are melted down, or vice versa. A turbine-shaft blank produced in this way may be brought to the appropriate radii of the first region and of the second region, for example by forging. The heat treatment of a combined turbine shaft produced by the ESR process may take place in a similar manner for the first region and the second region. A preheating treatment is carried out at about 1100xc2x0 C. for a period of about 26 hours and is continued by a furnace cooling to about 680xc2x0 C. This is followed, depending on the diameter of the shaft, by a quality heat treatment at the austenitizing temperature of about 1070xc2x0 C. for a period of about 33 hours. This is followed by tempering, for example for a period of about 24 hours, at a temperature of between 650xc2x0 C. and 680xc2x0 C., whereby it is possible to produce different tempering temperatures according to region.
In accordance with another mode of the invention, the first region is produced with a core region which is made of the second base material and extends around the axis of rotation by filling a hollow cylinder formed from the first base material with the second base material by melting down one or a number of electrodes. The hollow cylinder made of the first base material may be produced by conventional forging processes. When filling the hollow cylinder with the second base material or a third base material with high hot strength, for example through the use of the electro-slag remelting process (ESR process), the blank mold of the first region produced in this way can be welded to the solidifying ESR melt pool. It is also possible to overgrow the first region on the second region.
In accordance with a concomitant mode of the invention, the second region, the low-pressure part, is made by filling a hollow cylinder being formed of the second base material with the first base material or a further base material.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a turbine shaft and a method for producing a turbine shaft, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.